build: use consistent shebang line

Fixes #13948.

Change-Id: I37d734c1241f3d72d8fe33dfdf79b573e5476b1f
Reviewed-on: https://go-review.googlesource.com/18643Reviewed-by: Russ Cox <rsc@golang.org>

src/bootstrap.bash

-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright 2015 The Go Authors.  All rights reserved.
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.